1. Field of the Invention
This invention relates to a method and a device for applying prestressed, tension-proof reinforcing strips to constructions, the strips being fixed to the construction with an adhesive.
2. Description of Related Art
For many years, both research and practical work have been done to find a way of strengthening steel concrete constructions after completion by applying an additional reinforcement. The beginnings of this technology are described in a report by J. Bresson entitled xe2x80x9cNouvelles recherches et applications conxc3xa7emant l""utilisation des collages dans les structures Beton plaquxc3xa9xe2x80x9d, Annales ITBTP No. 278 (1971), Sxc3xa9rie beton, Beton armxc3xa9 No. 116. The technique dates back to the 1960s. Bresson concentrated on research into the bonding stresses in the vicinity of the anchorages of lamellar steel strips bonded to constructions with adhesive. One advantage is that over the last 25 years, engineers have been able to reinforce existing steel constructions such as bridges, bed-plates, overhead plates, longitudinal supports and the like by subsequently applying lamellar steel strips with adhesive. The reinforcing of concrete constructions by applying lamellar steel strips using e.g. epoxy resin adhesives is now considered a standard technology. Depending on the particular case in hand, the purpose of such a reinforcement is to: increase the working load and alter the static system by removing supporting elements such as pillars, or by reducing the supporting function of such elements and strengthen elements at risk from fatigue stress, increase rigidity compensate damage to the support system or renovate existing constructions, and effect post-construction reinforcement in the event of faulty calculation or execution of a particular construction
Post-construction reinforcement by means of applying lamellar steel strips with adhesive has been successfully used on numerous constructions, as described in, for example: Ladner, M., Ch.: xe2x80x9cGeklebte Bewehrung im Stahlbetonbauxe2x80x9d, Swiss Federal Laboratories for Materials Testing and Research (EMPA) Dxc3xcbendorf, Report No. 206 (1981); xe2x80x9cVerstxc3xa4rkung von Tragkonstruktionen mit geklebter Armierungxe2x80x9d, Schweizer Bauzeitung, special article in the 92nd year, volume 19 (1974); xe2x80x9cDie Sanierung der Gizenenbrxc3xccke xc3xcber die Muotaxe2x80x9d, Schweiz. Ingenieur and Architekt, special article in volume 41 (1980).
These conventional methods of reinforcement are, however, associated with certain disadvantages. Lamellar steel strips can only be supplied in short lengths, and hence only relatively short strips can be applied. This means that where there are lengthy spans, joints between the lamellae are unavoidable, thereby inevitably leading to potential weak spots. Furthermore, handling heavy lamellar steel strips on a building site is an awkward matter, and can cause considerable technical problems in the case of high-level constructions, or constructions which are otherwise difficult to access. In addition, there exists a risk of the steel rusting on the underside of the strips, even if corrosion protection treatment is carefully accomplished, i.e. of corrosion on the contact surface between the steel and the concrete, which can result in the strip becoming detached, and thus a loss of the reinforcement.
In the publication by U. Meier entitled xe2x80x9cBrxc3xcckensanierung mit Hochleistungs-Faserverbundwerkstoffenxe2x80x9d, published in Material+Technik, 15th year, volume 4 (1987), and in the dissertation by H. P. Kaiser, Dissertation ETH Zxc3xcrich (1989), the proposed remedy is to place the lamellar steel strips with carbon fibre reinforced epoxy resin lamellae. Lamellar strips made from this material are characterized by a low bulk density, very high strength, excellent endurance properties and outstanding resistance to corrosion. Instead of heavy lamellar steel strips one can, therefore, also use light, thin, carbon fibre reinforced plastic strips which can be transported to the construction site on virtually endless reels. Practical tests have shown that carbon fibre lamellae of 0.5 mm thickness can absorb the same amount of tensile force as the yield strength of a 3 mm thick FE360 steel strip.
Hence post-construction reinforcement with carbon fibre lamellae fixed directly onto the construction by means of adhesive is already a state-of-the-art technology. The method involving reinforcement with steel lamellae has now largely been replaced by the method whereby the construction is reinforced with non-prestressed carbon fibre lamellae.
It has proved advantageous, particularly when using fibre composite lamellae of the type suggested in ETH Dissertation No. 8918, such as e.g. carbon fibre lamellae, to additionally prestress these lamellae disposed on the concrete construction part, thereby improving the utility of the part and preventing the lamella from shearing off as a result of shear fractures in the concrete in the tension zone. The enormous elastic extensibility of carbon fibre lamellae represents a big opportunity for the aforementioned prestressing operation. The large elastic extensibility and the modulus of elasticity, which is adjusted to the particular circumstances, have a positive impact on prestress losses due to shrinkage and creep.
French Patent Reference 2,594,871 disclosed a method whereby a prestressed strip is applied to the structure to be strengthened, namely to reinforced concrete, and bonded to this structure with adhesive. During the process the strip is prestressed until the adhesive hardens. The device shown in FIGS. 6 and 7 for executing this method is merely a strap held in place by a metal plate, which strap is used to hold the strip in place. This presupposes the availability of rigid anchorage points for attaching these straps, but these are not, however, always provided in practice, and are not disclosed in French Patent Reference 2,594,871. Furthermore, the method disclosed in that document does not allow for the strip to be pressed against the structure at the same time as the bonding process, as is required to achieve reliable bonding.
One remaining difficult point is therefore the problem of anchoring the carbon fibre lamellae during the prestressing process, given that prestressing forces are of several tens of thousands of N. These enormous forces have to maintain the lamella to be applied under tension against the construction itself, at least until the adhesive has hardened completely.
One object of this invention is to provide a method for applying tension-proof reinforcing strips to constructions which, irrespective of the availability of anchoring points on the construction for absorbing stressing forces, will allow the reinforcing strip to be prestressed and then applied, and which is reliable, simple and inexpensive to use. Another object of this invention is to provide a compact, simple, reliable device for executing this method, which is also inexpensive to manufacture.
This object is achieved with a method for applying prestressed, tension-proof reinforcing strips to constructions in which the strip to be applied is prestressed, pre-treated with adhesive and then positioned up to a construction and bonded to this structure. The method of this invention requires no anchorage points on the construction for absorbing stress forces because it is positioned up to the construction by a device on which the strip can be stretched under prestressing force, such as a device used to press the strip against the corresponding, pre-treated part of the construction until the adhesive hardens. The task is also solved with an apparatus for executing this method, as described in this specification and in the claims.